B737 Captain reported a SOURCE OFF light illuminated during cruise. When running the checklists; the flight crew attempted a start of the APU; subsequently lost many systems and indications; and elected to make a precautionary landing.

Narrative

We had SOURCE OFF light on the right side came on just before top of descent. We followed the checklist procedure and were not able to get power back to the bus.Wanted to add some more information to the report.When running the check list procedure we attempted to put the gen back on the bus [and] the autopilot kicked off; the next step called for the APU. As soon as we put the APU gen on; the entire overhead panel back lighting went dark and could not tell if the APU was on the bus. The test light switch for the overhead lights functioned but once back in normal position it was dark. Had to turn on the bright overhead lights for the remainder of the flight. At about that time we lost flight directors; VNAV and LNAV; copilots PFD; ND and FMC; lost power to one fuel pump in the left tank and one also in the right; lost auto throttle; auto trim had to use manual trimming and the temperature in the forward cabin including the cockpit got extremely hot and uncontrollable. When configuring flaps for landing the forward flap position indicator was inoperative. After landing we can hear a horn in the av compartment associated with a low battery.Earlier on in the flight we had lost VHF 1 and were using VHF 2 for ATC. Sent a message to Dispatch to call me and to also get Maintenance Control on the call. We attempted to communicate on VHF 1 but it was impossible. Requested priority handling and had the trucks standing by and landed safely.All four flight attendants were brand new three of them on their first trip they did very well.